Mutagen-sensitive (mus) mutants of Drosophila melanogaster are being isolated and characterized for potential alterations in spontaneous and induced mutation frequencies and defects in DNA repair ability. Current studies employ selected monofunctional alkylating agents, which vary with respect to Swain-Scott constants, and include methyl methanesulfonate, ethyl methanesulfonate, methyl nitrosourea and ethyl nitrosourea. The sex-linked recessive lethal test and the X-linked white eye color locus are being employed to monitor the effect of specific mus loci on alkylation-induced mutation production. Current experiments are focused on the effects of the excision-defective mei-9 and mus(2)201 loci. In vitro cell culture systems prepared from embryos of wild type and mus strains are being employed to study both normal DNA synthetic capacity and DNA repair capability following damage by the alkylating agents. Repair studies utilize both autoradiographic determinations of unscheduled DNA synthesis (UDS) and alkaline and neutral sucrose gradient centrifugation to identify specific defects in the repair of alkylation-induced lesions.